The human monocytic leukemic cell line, THP-1, which differentiates toward macrophages in response to phorbol 12-myristate 13-acetate (PMA) was investigated for its ability to produce granulocyte colony-stimulating factor (G-CSF). G-CSF protein was neither produced during PMA-induced differentiation nor in response to dexamethasone (Dex) alone. However, when combined, PMA and Dex synergistically stimulated THP-1 cells to produce G-CSF. The synergistic interaction between PMA and Dex on G-CSF production appeared to be mediated through the production of interleukin-1 (IL-1) since neutralization of IL-1 activity completely inhibited G-CSF production. Further experiments demonstrated that in THP-1 cells pretreated with PMA, Dex potently synergized with IL-1 to stimulate G-CSF production.